Re: [PATCH v3 1/1] block/blkio: use qemu_open() to support fd passing for virtio-blk

[Stefano Garzarella]

On Thu, May 11, 2023 at 11:03:22AM -0500, Jonathon Jongsma wrote:
> On 5/11/23 4:15 AM, Stefano Garzarella wrote:
> > The virtio-blk-vhost-vdpa driver in libblkio 1.3.0 supports the new
> > 'fd' property. Let's expose this to the user, so the management layer
> > can pass the file descriptor of an already opened vhost-vdpa character
> > device. This is useful especially when the device can only be accessed
> > with certain privileges.
> >
> > If the libblkio virtio-blk driver supports fd passing, let's always
> > use qemu_open() to open the `path`, so we can handle fd passing
> > from the management layer through the "/dev/fdset/N" special path.
> >
> > Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
> > ---
> >
> > Notes:
> >     v3:
> >     - use qemu_open() on `path` to simplify libvirt code [Jonathon]
>
>
> Thanks
>
> The one drawback now is that it doesn't seem possible for libvirt to 
> introspect whether or not qemu supports passing an fd to the driver or 
> not.

Yep, this was because the libblkio library did not support this new way.

> When I was writing my initial patch (before I realized that it was 
> missing fd-passing), I just checked for the existence of the 
> virtio-blk-vhost-vdpa device. But we actually need to know both that 
> this device exists and supports fd passing.

Yep, this was one of the advantages of using the new `fd` parameter.
Can't libvirt handle the later failure?

> As far as I can tell, versions 7.2.0 and 8.0.0 include this device but 
> won't accept fds.

Right.

How do you suggest to proceed?

Thanks,
Stefano